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Auckland's $220 million motorway-to-motorway connection through Manukau is expected to be open in both directions from Sunday, but local traffic faces considerable disruption while a long off-ramp is built.

The Transport Agency, which opened the 4.5km connection's eastbound lanes last month, has since completed its westbound carriageway between State Highways 1 and 20.

But now it needs to cut off Roscommon Rd from the Southwestern Motorway while it realigns a 150m section of Puhinui Stream to enable it to build an off-ramp between the new link's Cavendish and Puhinui interchanges, a project expected to take until late December.

Although some traffic will be able to use a detour under the motorway and along newly built Noel Burnside Rd to reach Puhinui Rd, the agency says that route could become very congested and should be reserved for drivers wanting to reach the Manukau Memorial Gardens cemetery or the airport via State Highway 20B.

It has briefed funeral directors and is placing advertisements in the Herald to ensure mourners do not get lost.

Up to 1800 vehicles an hour now travel northbound along Roscommon Rd in the morning traffic peak, and the agency says drivers wanting to reach the motorway should do so by turning right up Wiri Station Rd and then left at a new interchange at Lambie Drive.

It advises drivers unfamiliar with the area to travel north along the motorway as far as George Bolt Drive to reach the airport, until the direct exit through the Cavendish and Puhinui interchanges is completed.

Transport Agency state highways manager Tommy Parker has appealed for patience, promising a much faster trip to the airport by Christmas.

A 240-metre flyover at the Manukau end of the project will open on Sunday for traffic from north to west, but vehicles will have to go through signals on Redoubt Rd to reach it before a full westbound connection between the two motorways is ready on October 23.

Although ramp signals will moderate traffic flows off the flyover, vehicles joining SH1 from SH20 have free passage.